The finding of these extensive lacustrine deposits naturally opened up a number of questions of the greatest interest and importance. To what level had the waters of the lake attained; what were its limits horizontally; and, most important of all, at what period had it existed, and what were its relations to the ancient monuments of the oasis? With the object of solving these problems it has been my endeavour to collect all possible information concerning the deposits, and, although it is perhaps too soon to draw inferences with any great certainty, I propose to put on record such data as I have been able to obtain, and to indicate the conclusions to which they appear to lead.
It is, of course, obvious that the lake was, geologically speaking, of comparatively recent date; the lacustrine deposits have nothing in common with those of the Cretaceous and Eocene formations which build up the plateau-massif out of which the oasis-depression is hewn. The sands and clays are much softer and less consolidated beds, laid down superficially and unconformably on the uneven surfaces of the older formations; the depression had, in fact, almost attained to its modern dimensions before the beds in question were formed. My first impression was that the lake dated from prehistoric, if not prehuman, times; that it existed before the prevailing desert conditions set in, when the Nile Valley was the site of extensive lakes and the country was partially wooded. The existence of thick deposits of calcareous tufa on the upper portions of the cliffs of the depression suggested a considerable outpouring of water from springs, and it seemed not improbable that the surplus water collected on the floor of the depression below.
For some time I was unable to discover any clue to the age of the deposits, in the shape of organic remains or human relics. Then, in a pit sunk for surface-water in the neighbourhood of Headquarters,[8] some fragments of pottery were found at the base of the deposit. This discovery was followed by the finding of other pieces of pottery firmly embedded in some of the clay hummocks to the south-west of Headquarters. These were without question in situ, and proved that the lake was contemporaneous with man.
Further search, in which I was greatly assisted by my friend H. H. Baker, M.B., led to the discovery of human settlements, apparently on the margins of the lake. In these localities large quantities of broken pottery were found associated with the bones of domesticated animals, while the fresh-water gastropod shell, Melania tuberculata, was found to be abundant in some of the beds. Careful exploitation of one or two of these sites, about 6 kilometres south-west of Headquarters, enabled us to procure a number of complete earthenware vessels, the chief types of which are shown in one of our illustrations. A comparison of these with pottery of known age must be left until we have referred to the horizontal and vertical extension of the lake.
Judging from the still existing deposits, and utilizing (in the northern part of the oasis) a considerable number of levels, I believe the limits of the lake to have been as shown on the accompanying plan. From its most northerly point in lat. 25° 45′ N., in the neighbourhood of the modern Ain el Ghazâl, its western boundary trended S.S.W., passing about 1½ kilometres west of Meheriq village to the south-west end of Jebel Têr; thence its shore-line lay at the foot of Jebel Tarwan, rounding which it projected somewhat into the Bellaida district between Jebel Têr and Jebel Tarif. South of this it bore slightly west of south, passing about 2½ and 3 kilometres west of Kharga and Gennâh villages respectively. From the latter point the shore-line proceeded almost due south to lat. 25° 2′ N., in the vicinity of Ain Girm Meshîm.
Its eastern limit appears to have been about a kilometre east of Ain el Qasr, and some three times that distance east of Meheriq, so that on this latitude the lake had a breadth of 4½ kilometres. In the neighbourhood of Headquarters its margin lay 2 kilometres to the east, whence it ran almost due south for about 12 kilometres. Here, just to the south-west of Ain Harrân, the lake attained its greatest width—i.e., 15 or 16 kilometres, gradually diminishing southwards to Ain Girm Meshîm. The total length of this portion of the lake was just over 80 kilometres.
Both at Ain Girm Meshîm and Ain el Doum the lacustrine deposits are well seen; over the intermediate country they were not observed, and as this area lies at a somewhat high elevation, it seems probable that there was a break in the continuity of the lake of about 15 kilometres in extent.
The southern portion appears to have measured approximately 45 by 15 kilometres, its long axis lying along a line passing through Ain el Doum, Beris, and Maks Qibli. To the south-east the lake had a local extension in the neighbourhood of Dush. It is thus seen to have extended, with a possible break of a few kilometres in the district near Ain Girm Meshîm, almost throughout the length of the depression, over a distance of 136 kilometres, or 85 miles.
It proved a matter of some difficulty to determine the maximum height to which the lake had reached, as in most localities the deposits have suffered considerable denudation, and their margins are usually obscured by blown sand and superficial detrital material. The pottery to the south-west of Headquarters was ascertained to occur at 47 metres above sea-level, the uppermost limit of the deposits in that area being 62 metres. At Headquarters the lake clays occur up to between 65 and 66 metres; at Ain Terfai and Ain Mahmud, north of Meheriq, to 65 and 67 metres respectively. Still farther north I found there were stretches of similar clays at 76 metres near Ain el Qasr, and at 84 to 85 metres in the neighbourhood of Ain el Ghazâl.
On the west side a very well-defined plain formed of the lake beds occurs immediately to the east of the Necropolis, and was found to lie at 70 metres above sea-level. But the most valuable data of all were those obtained at the Gorn el Gennâh. There, as anticipated, the lacustrine deposits, containing Melania and Limnæa in abundance, were found to be well developed and exposed, being piled up on the flanks of the hill on its north, east, and south sides, the exact limits of the beds being best seen on the south-east side near the ruins of a columbarium. Immediately behind the latter the top of the stratified clays was determined as 66½ metres, a figure agreeing remarkably closely with the majority of the maximum heights farther north. Still higher up, however, are other clays, which, while similar in general appearance, are either very irregularly bedded or altogether unstratified. The upper limit of these clays was found to be 82·76, or, say, 83 metres. While these highest beds may possibly have been deposited by the waters of springs issuing from faults and fissures (the line of disturbance mentioned in a previous chapter passes through the Gorn at this point), one must, in the absence of any definite evidence to the contrary, regard them as probably representing the extreme marginal deposits of the lake.